One of the serious challenges the industry faces is developing new gate dielectric materials for Dynamic Random Access Memory (DRAM) and capacitors. For decades, silicon dioxide (SiO2) was a reliable dielectric, but as transistors have continued to shrink and the technology moved from “Full Si” transistor to “Metal Gate/High-k” transistors, the reliability of the SiO2-based gate dielectric is reaching its physical limits. The need for new high dielectric constant material and processes is increasing and becoming more and more critical as the size for current technology is shrinking. New generations of oxides especially based on lanthanide-containing materials are thought to give significant advantages in capacitance compared to conventional dielectric materials.
Nevertheless, deposition of lanthanide-containing layers is difficult and new material and processes are increasingly needed. For instance, atomic layer deposition (ALD) has been identified as an important thin film growth technique for microelectronics manufacturing, relying on sequential and saturating surface reactions of alternatively applied precursors, separated by inert gas purging. The surface-controlled nature of ALD enables the growth of thin films having high conformality and uniformity with an accurate thickness control. The need to develop new ALD processes for rare earth materials is obvious.
Unfortunately, the successful integration of compounds into deposition processes has proven to be difficult. Two classes of molecules are typically proposed: beta-diketonates and cyclopentadienyls. The former family of compounds is stable, but the melting points always exceed 90° C., making them impractical. Lanthanide 2,2-6,6-tetramethylheptanedionate's [La(tmhd)3] melting point is as high as 260° C., and the related lanthanide 2,2,7-trimethyloctanedionate's [La(tmod)3] melting point is 197° C. Additionally, the delivery efficiency of beta-diketonates is very difficult to control. Non-substituted cyclopentadienyl compounds also exhibit low volatility with a high melting point. Molecule design may both help improve volatility and reduce the melting point. However, in process conditions, these classes of materials have been proven to have limited use. For instance, La(iPrCp)3 does not allow an ALD regime above 225° C.
Some of the lanthanide-containing precursors currently available present many drawbacks when used in a deposition process. For instance, fluorinated lanthanide precursors can generate LnF3 as a by-product. This by-product is known to be difficult to remove.
Consequently, there exists a need for alternate precursors for deposition of lanthanide-containing films.